The avian hypothalamus exerts a principally stimulatory influence on prolactin (PRL) secretion. Several lines of evidence support vasoactive intestinal peptide (VIP) as the most important prolactin-releasing factor (PRF) in birds. One vasoactive intestinal octacosapeptide is produced naturally in chickens and may be referred to as chicken VIP or cVIP. This particular VIP has the amino acid chain of His-Ser-Asp-Ala-Val-Phe-Thr-Asp-Asn-Tyr-Ser-Arg-Phe-Arg-Lys-Gln-Met-Ala-Va l-Lys-Lys-Tyr-Leu-Asn-Ser-Val-Leu-Thr. cVIP is cross-reactive with any bird of the Avian species, including turkeys and ducks. In other words, turkeys and ducks also recognize cVIP.
VIP meets many of the qualifications of a PRF. For example, VIP stimulates PRL release from the anterior pituitary in vitro, and the response is closely correlated to the reproductive state of the animal. VIP also stimulates PRL release in vivo in median eminence-deafferentated hens (see, Opel et al., Proc. Soc. Exp. Biol. Med., 187, 455 (1988)). Also, L. J. Mauro et al., Gen. Comp. Endoc., 87, 481 (1992) reported the presence of high VIP concentration in median eminence, particularly the external layer. The presence of high affinity VIP receptors on the anterior pituitary cells has also been reported.
Further evidence that VIP is a PRF is provided by recent findings that lesioning of VIP cell bodies in the infundibular nuclear complex (INF) eliminates PRL increases associated with the photo-induced reproductive cycle and suppresses elevated PRL associated with incubation behavior. Finally, immunoneutralization of turkeys with endogenous VIP reduced both circulating PRL and pituitary PRL mRNA, totally blocked the PRL release effected by electrical stimulation of the medial preoptic nucleus, and blocks the hormonal and behavioral characteristics of incubating hens.
For example, the active immunization of turkey hens with VIP conjugated to an adjuvant protein was found to increase egg production. Although the bio-mechanical mechanism is not fully understood, it is believed that the antibodies so produced complex the turkey hen's naturally produced VIP. This naturally produced VIP regulates the secretion of the hormone prolactin from the pituitary gland of the turkey hen. In turn, increased prolactin secretion causes broodiness in turkey hens. Broodiness, in turn, is one of the factors that may lead to poor egg production. Accordingly, egg production is enhanced by binding the turkey hen's naturally produced VIP with the natural antibodies generated by the turkey before the turkey's VIP can act upon the turkey's pituitary gland to increase prolactin secretion.
Because of the prominent role VIP plays in the regulation of PRL secretion, it is likely that the secretion of PRL is mediated through changes in VIP secretion and/or gene expression, which may vary between the hypothalamic and hyperolactenemic birds. Therefore, a need exists to isolate, identify and regulate the structural gene encoding turkey VIP.